This study aims to investigate a novel homogeneous polysaccharide (UMRA) extracted from Urtica macrorrhiza Hand.-Mazz. in Yunnan, China, focusing on characterizing its primary structure and preliminarily exploring its immunological activity. UMRA was purified through a series of steps, including water extraction, alcohol precipitation, quaternary ammonium salt precipitation, and gel column chromatography. Its purity and chemical structure were analyzed using high-performance gel permeation chromatography, ultraviolet spectroscopy, fourier-transform infrared spectroscopy, methylation, and nuclear magnetic resonance spectroscopy. The immunomodulatory activity of UMRA was evaluated by assessing its effects on macrophage viability and cytokine secretion. The results demonstrated that UMRA is a homogeneous acidic polysaccharide with a weight-average molecular weight of approximately 37.66 kDa. It comprised with rhamnose, galacturonic acid, galactose, glucuronic acid, arabinose, and mannose in a molar ratio of 3.94∶3.34∶2.26∶1.00∶0.95∶0.46. Further structural analysis revealed that UMRA possesses a pectic backbone with an rhamnogalacturonan-I  framework, featuring a repeating [→2)-α-L-Rhap-(1→4)-α-D-GalpA-(1→] disaccharide unit as its main chain, with side chains consisting of galactose, arabinose and glucuronic acid. UMRA exhibited no cytotoxicity toward macrophages at concentrations ranging from 0.4 to 250 μg/mL and promoted macrophage secretion of nitric oxide (NO), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6). These findings provide a foundation for the structural elucidation and potential pharmaceutical development of polysaccharides derived from U. macrorrhiza.